in order to feel out timber joinery to see if it's something i can attempt on a large (house) scale, i'd like to see a smaller project through.

my plan is to simply halve the design in the back of steve chappell's "a timber framers workshop" which would leave me with a 12x12 structure roughly 11 ft. high. in order to keep the math simple i'd like to just halve the timber dimensions as well but it gets silly when the braces will be 1.5 inches thick.

so am i just building a model here or will it stand up to use?

Without looking at any numbers, halving may not be the way to go. The reason is that the strength of a piece of wood is not in a direct mathematical relationship to its size, rather it is a functional relationship. In other words, a 4x4 is not half the strength of an 8x8 (consider the fact that a 4x4 square has 1/4, not 1/2, or the area of an 8x8 square) And as you said, 1 1/2 inch braces are kind of silly. I might be wrong (as I said, I haven't mathematically figured it out, just goin off of intuition) but I would think you would want something in the range of 2/3 to 3/4 of the original timber size, not half.

Why not build a pergola or garden shed? Real size timbers, real joinery.

good call, i've actually got a landscaping client wanting a pergola and what plans i've found tend to reflect the kind of HGTV "get them the invoice before the whole thing falls apart" design that gets under my skin. yeah, a pergola would be just the thing...course if i'm not as good at this as i hope to be...& the misses gets a wonky pergola! still would like to do a mini- high ost cape though just to go through the process...i don't want to have to explain why somebody's pergola has a pitched roof on it.

You can always start with framed sawhorses. They are simple, and give you the chance to try a few simple mortise and tenon joints. If you like the work, then you have a nice sturdy set of horses to work on.

I met someone who was building timber framed chicken tractors out of some small wood. I never did hear how they worked out for him.